Nowadays, thanks to the technological developments in the field, it is becoming more convenient in many applications, such as also in the "consumer" sector, to use a three-phase motor in place of the more traditional, so-called universal motor. In particular, the use of three-phase motors is rapidly spreading to domestic appliances. The typical characteristic curve of a three-phase motor is shown in FIG. 1, while the characteristic curve that could be obtained by exerting a correct control of the operating conditions is shown in FIG. 2.
As it may be observed, a generic three-phase motor has a high torque at start-up, a low torque at high speeds, a high slip and a relatively low efficiency. In contrast, an equivalent motor when electronically controlled has a reduced slip, a high efficiency and can provide for a high torque also at high speeds.
To implement an accurate control of a three-phase motor, beside controlling the stator frequency, it is also necessary to implement an effective control of the slip, which represents the difference between the frequency of the stator and the frequency of the rotor (which corresponds to the motor rotation speed). This control is essential if the motor is to be employed to the limits of its capabilities, that is, to the maximum of its performances. Indeed, under extreme conditions a precisely controlled functioning is very important. To optimize efficiency, it is necessary that the operating point of the motor be precisely controlled. A slip too small, as well as a slip too large, may cause a decrease of the efficiency and make attaining top performances very difficult.
Slip is commonly controlled by the use of a microprocessor storing in a look-up table a series of permitted slip values (usually expressed in percentages of the stator frequency). Unfortunately, using a look-up table, that is, a set of pre-defined regulation values, is limiting because it does not allow for a dynamic self-adapting control upon changing conditions of operation of the motor.